1. Field of the Invention
This invention relates in general to giant magnetoresistive (GMR) heads, and more particularly to a method and apparatus for performing spin valve combined pinned layer reset and hard bias initialization at the HGA level.
2. Description of Related Art
An MR sensor detects magnetic field signals through the resistance changes of a magnetoresistive element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the element. The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the element resistance varies as the square of the cosine of the angle between the magnetization in the element and the direction of sense or bias current flow through the element.
MR sensors have application in magnetic recording systems because recorded data can be read from a magnetic medium when the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in an MR read head. This in turn causes a change in electrical resistance in the MR read head and a corresponding change in the sensed current or voltage.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. This GMR effect has been found in a variety of systems, such as Fe/Cr or Co/Cu multilayers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers, as well as in essentially uncoupled layered structures in which the magnetization orientation in one of the two ferromagnetic layers is fixed or pinned.
A particularly useful application of GMR is a sandwich structure comprising two essentially uncoupled ferromagnetic layers separated by a nonmagnetic metallic spacer layer in which the magnetization of one of the ferromagnetic layers is xe2x80x9cpinnedxe2x80x9d. The pinning may be achieved by depositing the ferromagnetic layer to be pinned onto an antiferromagnetic layer, such as an iron-manganese (Fexe2x80x94Mn) layer, to create an interfacial exchange coupling between the two layers. The spin structure of the antiferromagnetic layer can be aligned along a desired direction (in the plane of the layer) by heating beyond the xe2x80x9cblockingxe2x80x9d temperature of the antiferromagnetic layer and cooling in the presence of a magnetic field. The blocking temperature is the temperature at which exchange anisotropy vanishes because the local anisotropy of the antiferromagnetic layer, which decreases with temperature, has become too small to anchor the antiferromagnetic spins to the crystallographic lattice. The unpinned or xe2x80x9cfreexe2x80x9d ferromagnetic layer may also have the magnetization of its extensions (those portions of the free layer on either side of the central active sensing region) also fixed, but in a direction perpendicular to the magnetization of the pinned layer so that only the magnetization of the free-layer central active region is free to rotate in the presence of an external field. The magnetization in the free-layer extensions may be fixed by longitudinal hard biasing or exchange coupling to an antiferromagnetic layer. However, if exchange coupling is used the antiferromagnetic material is different from the antiferromagnetic material used to pin the pinned layer, and is typically nickel-manganese (Nixe2x80x94Mn). This resulting structure is called a xe2x80x9cspin valvexe2x80x9d (SV) MR sensor. In a SV sensor only the free ferromagnetic layer is free to rotate in the presence of an external magnetic field. U.S. Pat. No. 5,159,513, assigned to IBM, discloses a SV sensor in which at least one of the ferromagnetic layers is of cobalt or a cobalt alloy, and in which the magnetizations of the two ferromagnetic layers are maintained substantially perpendicular to each other at zero externally applied magnetic field by exchange coupling of the pinned ferromagnetic layer to an antiferromagnetic layer. U.S. Pat. No. 5,206,590, also assigned to IBM, discloses a basic SV sensor wherein the free layer is a continuous film having a central active region and end regions. The end regions of the free layer are exchange biased by exchange coupling to one type of antiferromagnetic material, and the pinned layer is pinned by exchange coupling to a different type of antiferromagnetic material.
Preferably, the thickness of the spacer layer is less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by interfaces between the spacer layer and the pinned and free layers. When the magnetizations of the pinned and free layers are substantially parallel, scattering is minimal and the electrical resistance of the sensor is at a minimum. When the magnetizations of the pinned and free layers are substantially antiparallel, scattering is maximized and the electrical resistance of the sensor is at a maximum. Changes in scattering alter the electrical resistance of the spin valve sensor in proportion to sin xcex8, where xcex8 is the angle between the magnetizations of the pinned and free layers. A spin valve sensor is characterized by a magnetoresistive (MR) coefficient (the ratio of the change in electrical resistance of the sensor to its maximum electrical resistance) that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a xe2x80x9cgiant magnetoresistivexe2x80x9d (GMR) sensor.
The physical origin is the same in all types of GMR structures: the application of an external magnetic field causes a variation in the relative orientation of the magnetizations of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
SV sensors are a replacement for conventional MR sensors based on the AMR effect. They have special potential for use as external magnetic field sensors, such as in anti-lock braking systems, and as read heads in magnetic recording systems, such as in rigid disk drives.
A read head employing a spin valve sensor (hereinafter, a xe2x80x9cspin valve read headxe2x80x9d) may be combined with an inductive write head to form a combined magnetic head. In a magnetic disk drive, an air bearing surface (ABS) of a combined magnetic head is supported adjacent a rotating disk to write information on or read information from the disk. Information is written to the rotating disk by magnetic fields which fringe across a gap between the first and second pole pieces of the write head. In a read mode, the electrical resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields on the rotating disk. When a sense current Is is conducted through the spin valve sensor, electrical resistance changes cause potential changes that are detected and processed as playback signals.
Another type of spin valve sensor, an antiparallel (AP) pinned spin valve sensor, is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin, which is incorporated into this application by this reference. The AP pinned spin valve sensor differs from the pinned layer spin valve sensor, described above, in that the pinned layer of the AP pinned spin valve sensor comprises multiple thin films, which are collectively referred to as an antiparallel (AP) pinned layer, while the pinned layer of the pinned layer spin valve sensor is a single thin film layer. The AP pinned layer has a nonmagnetic spacer film, hereinafter referred to as an antiparallel (AP) coupling film, sandwiched between first and second ferromagnetic thin films. The first thin film is exchange coupled to the pinning layer by being immediately adjacent thereto, and has its magnetic moment directed in a first direction. The second thin film is immediately adjacent to the free layer and is exchange coupled to the first thin film by the minimal thickness (in the order of 5 .ANG.) of the AP coupling film between the first and second thin films. The magnetic moment of the second thin film is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first film. The magnetic moments of the first and second films subtractively combine to provide a net pinning moment of the pinned layer. The direction of the net moment is determined by the thicker of the first and second thin films. The thicknesses of the first and second thin films are chosen to reduce the net moment. A reduced net moment results in a reduced demagnetization (demag) field from the AP pinned layer. Since the exchange coupling between the pinning layer and first thin film is inversely proportional to the net pinning moment, the exchange coupling is increased.
A transfer curve (a plot of the readback signal of the spin valve head versus the applied signal from the magnetic disk) of a spin valve sensor is defined by sin xcex8. A substantially flat portion of the transfer curve is selected for location of a bias point so that response of the sensor is substantially linear. Since positive and negative magnetic fields from a moving magnetic disk are typically equal in magnitude, it is important that positive and negative changes in the magnetoresistance of the spin valve sensor about the bias point on the transfer curve also be equal, which is referred to herein as read signal symmetry. The location of the bias point on the transfer curve is influenced by various magnetic fields when the sensor is in a quiescent state (sense current conducted, but an absence of magnetic fields from the rotating disk). When these magnetic fields are not balanced there will be read signal asymmetry in a positive or a negative direction with respect to the bias point.
A high performance spin valve head has high magnitude read signal output, and low, or no, read signal asymmetry. Where there is no read signal asymmetry, the transfer curve of the read signal is centered about a zero bias point. This means that from a point where the input signal is zero, the amplitudes of the positive and negative read signal outputs are equal as the input signals go between positive to negative levels. The level of performance of the spin valve head is dependent upon proper orientation and magnetizations of the aforementioned layers. If either of the pinned or biasing layers acquires a multi-magnetic domain state, read signal output will be decreased and read signal asymmetry will be increased. The impact on read signal output and read signal asymmetry will be even greater if the magnetic spins of the pinning layer are disoriented.
Furthermore, the SV sensor, which is typically fabricated by depositing an antiferromagnetic layer of Fexe2x80x94Mn onto the ferromagnetic pinned layer of cobalt (Co) or permalloy (Nixe2x80x94Fe), suffers from the problem that the range of blocking temperature for this interface is relatively low, i.e., it extends only from approximately 130xc2x0 C. to approximately 160xc2x0 C. These temperatures can be reached by certain thermal effects during operation of the disk drive, such as an increase in the ambient temperature inside the drive, heating of the SV sensor due to the bias current, and rapid heating of the SV sensor due to the head carrier contacting asperities on the disk. In addition, during assembly of the disk drive the SV sensor can be heated by current resulting from an electrostatic discharge. If any of these thermal effects cause the SV sensor to exceed the antiferromagnet""s blocking temperature the magnetization of the pinned layer will no longer be pinned in the desired direction. This will lead to a change in the SV sensor""s response to an externally applied magnetic field, and thus to errors in data read back from the disk.
U.S. Pat. No. 5,650,887, commonly assigned to IBM, discloses a recovery system and process to reset the magnetization of the SV sensor""s pinned layer to the desired orientation. A pinned layer magnetization reset system is incorporated into systems that use SV sensors. The reset system generates an electrical current waveform that is directed through the SV sensor with an initial current value sufficient to heat the antiferromagnetic layer above its blocking temperature, and a subsequent lower current value to generate a magnetic field around the pinned layer sufficient to properly orient the magnetization of the pinned layer while the antiferromagnetic layer is cooling below its blocking temperature. This process resets the magnetization of the pinned layer to its preferred orientation and returns the SV sensor response substantially back to its desired state. However, U.S. Pat. No. 5,650,887 only resets the pinned layer.
U.S. Pat. No. 5,974,657 discloses a system for resetting of the magnetization of the hard biasing layer and the antiferromagnetic pinning layer at the row level. The antiferromagnetic pinning layer is reset by heating the pinning layer with a current pulse conducted through the leads to the conductive layers of the spin valve head so that localized heating takes place adjacent the pinning layer as contrasted to ambient heating of the spin valve head. Simultaneous with the localized heating the first magnetic field is applied for orienting the magnetic spins of the pinning layer perpendicular to the ABS and resetting the magnetic moment of the pinned layer perpendicular to the ABS in a single domain state. Subsequently, a second magnetic field is applied for resetting the magnetic moment of the hard biasing layer parallel to the ABS in a single domain state. However, U.S. Pat. No. 5,974,657 does not disclose a system that will automatically perform a process for resetting the pinned layer and the hard bias layer at the head gimbal assembly (HGA) level.
It can be seen that there is a need for a method and apparatus for performing automated spin valve combined pinned layer reset and hard bias initialization at the HGA level.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for performing spin valve combined pinned layer reset and hard bias initialization at the HGA level.
The present invention solves the above-described problems by combining a current pulse with an assisting magnetic field. The pinned layer reset and hard bias initialization is automated and performed by a single tool at the head gimbal assembly level to increase manufacturing throughput. The process may be automatically performed in response to a single user action.
A method and article of manufacture in accordance with the principles of the present invention includes initiating an automated process in response to a single user action, the process including first resetting the pinned layer of the spin valve sensor by loading a head gimbal assembly in a first orientation and applying a current pulse and a first assisting magnetic field of a magnet to the spin valve sensor and then reinitializing the hard bias layer of the spin valve sensor by rotating the head gimbal assembly 90 degrees to a second orientation and applying a second magnetic field of the magnet.
Other embodiments of a method and article of manufacture in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the resetting the pinned layer further comprises connecting the spin valve sensor to a pulse generator, positioning the head gimbal assembly in a first orientation within poles of a magnet, applying a current pulse to the spin valve sensor, turning on the magnet to produce a magnetic field for resetting the pinned layer of the spin valve sensor, turning the magnetic field of the magnet off and removing the head gimbal assembly from the magnet.
Another aspect of the present invention is that the reinitializing the hard bias layer further comprises rotating the head gimbal assembly 90 degrees to a second orientation, re-inserting the head gimbal assembly between the poles of the magnet, turning magnet on to reinitialize the hard bias layer of the spin valve sensor, turning the magnet off and removing the head gimbal assembly from the magnet poles of the magnet.
Another aspect of the present invention is that the first orientation aligns the pinned layer magnetization with the first assisting magnetic field of the magnet.
Another aspect of the present invention is that the second orientation aligns the hard bias layer magnetization with the second magnetic field of the magnet.
Another aspect of the present invention is that the magnetic field is 6500 Oersted.
Another aspect of the present invention is that the current pulse includes an adjustable current level.
Another embodiment of the present invention includes a system for performing automated spin valve combined pinned layer reset and hard bias initialization at the head gimbal assembly level. The system includes a motion controller for holding and controlling the motion of a head gimbal assembly having a spin valve sensor relative to a magnet, a pulse generator for applying a current pulse to the spin valve sensor and a processor, coupled to the motion controller, the pulse generator and the magnet, the processor in response to a user action controlling the motion controller to position of the head gimbal assembly, controlling the application of the current pulse to the spin valve sensor and controlling the application of a magnetic field of the magnet. The processor first resets the pinned layer of the spin valve sensor by loading the head gimbal assembly in a first orientation within the poles of the magnet and applying a current pulse from the pulse generator and a first assisting magnetic field of the magnet to the spin valve sensor and then reinitializes the hard bias layer of the spin valve sensor by rotating the head gimbal assembly 90 degrees to a second orientation and applying a second magnetic field of the magnet.